In this cross-sectional, epidemiological study, the iodine nutritional status and the prevalence of thyroid diseases were assessed in Jiangxi province, China. Among 2636 participants, a significant association was found between iodine status and thyroid diseases. It was revealed that adults who had a low or high UIC were inclined to develop thyroid diseases, suggesting that iodine nutrition played an important role in the progression of thyroid diseases.
Iodine is an important synthetic raw material for the synthesis of thyroxine, which is closely associated with human health, especially for the brain and central nervous system. Both iodine deficiency and excessive iodine lead to thyroid diseases [6]. Median UIC, a validated biomarker for assessment of iodine status, facilitated monitoring of thyroid function [7]. In the present study, it was found that the median of UIC was 176.4 µg/L, which reflected a normal state of iodine consumption in Jiangxi province. Moreover, 14.4%, 44.5%, 26.1%, and 15.0% of participants had deficiency, adequate, more than adequate, and excessive iodine concentrations, respectively. The results of the present study were consistent with those previously reported in China mainland [8, 9], which could be related to the implementation of the USI policy in China.
Moreover, in the present survey, it was indicated that the incidence rates of hyperthyroidism, subclinical hyperthyroidism, hypothyroidism, and subclinical hypothyroidism were 0.91%, 0.57%, 0.34%, and 7.89%, respectively. Similar to studies conducted by Jin et al. and Wan et al. [10, 11], subclinical hypothyroidism was also found as the most common thyroid dysfunction in the present study. Regrettably, mechanisms that iodine induced subclinical hypothyroidism have still remained elusive. Two assumptions were presented in the following. Firstly, chronic iodine stimulation could be a factor for iodine-induced hypothyroidism because of the Wolff − Chaikoff effect[12]. Secondly, a high iodine intake aggravated thyroid inflammatory response, which could be related to the role of IP10 and CD4+ T lymphocytes [13].
According to previous studies [8–11], different statuses of thyroid dysfunction were related to gender and iodine nutritional status. The USI has been implemented for more than two decades in China, and the iodine nutrition and thyroid dysfunction have been remarkably changed. A prevalence of 7.89% in subclinical hypothyroidism was found in the present study, while a higher prevalence (up to 16.7%) was reported by Shan et al. [14]. Dismissing iodine deficiency and different populations could be possible explanations for the higher incidence of subclinical hypothyroidism. In the present study, subjects in the excessive iodine group were at a higher risk of thyroid dysfunction compared with those in the adequate iodine group, which was consistent with Shan et al.’s findings [14].
The incidence of thyroid nodules was 9.45% in the present study, which was slightly lower than that in the coast of China (15.5%) [15] and Germany (23.4%) [16]. Several epidemiological studies have demonstrated that the incidence of thyroid nodules increased with age because of degenerative changes in thyroid [16, 17]. In the present study, it was found that the incidence of thyroid nodules was elevated with increasing iodine level. Over 3 times risk in excessive iodine group would lead to development of thyroid nodules compared with adequate iodine group. However, the results were not perfectly consistent with those of a previous study, in which the relationship of iodine level and the incidence of thyroid nodules presented a U-shaped curve [19]. The explanation of epidemiological difference was that IDD was almost eradicated after implementation of the USI policy for more than 25 years. Moreover, several studies reported that the risk of thyroid nodules increased when UIC was > 400 µg/L[18, 20]. There was no exact mechanism regarding excessive iodine leading to thyroid nodules, and additional studies are needed to provide direct evidence.
TAI is frequently diagnosed in cases with positive thyroid antibodies. In the present study, the incidence of 12.7% in TAI cases was similar to previously reported data [21, 22]. Compared with adequate iodine status, individuals with iodine deficiency, more than adequate, and excessive iodine were at a higher risk of TAI. A U-shaped relationship between iodine level and TAI in adults was found in previous studies [23, 24], which was consistent with the results of the present study. Furthermore, a strong correlation was noted between TAI and iodine status. Although the mechanisms that iodine-induced abnormal thyroid antibodies have still remained elusive, some hypotheses have been raised. Firstly, excessive iodine intake increased thyroglobulin immunogenicity and risk of TAI by unmasking a cryptic epitope on thyroglobulin, which initiated the autoimmune process during development of TAI [25]. Secondly, excessive iodine was resulted in oxidative stress, lipid oxidation, and thyroid tissue injuries [26]. Finally, the direct stimulatory effect of iodine on immune cells could be involved in the development of TAI [13].
Firstly, in the four models, between the thyroid dysfunction and UIC, subjects with excessive iodine were at a risk of thyroid dysfunction. In the model 2, After adjusting for age and gender, the risk of thyroid dysfunction had no significant association with the excessive iodine, which could be different from Deng et al.’s findings[27], and could be attributed to the involvement of elderly subjects in Deng et al.’s study. Secondly, the relationship between the incidence of thyroid nodules and UIC is noteworthy. As shown in Table 4, the risk of thyroid nodules exhibited a 21% reduction in models 1 and 4 in the excessive iodine group, which indicated that thyroid antibodies could play an important role in the incidence of thyroid nodules [28]. Thirdly, in models 1, 2, 3, and 4, the ORs of TAI patients almost remained unchanged after adjusting for the risk factors in iodine deficiency and excessive iodine groups, which suggested that variables were confounding for the TAI, and the iodine level was fatal for TAI [23]. However, the underlying mechanisms should be elucidated in the future studies.
Limitations
This epidemiological study had some limitations. This cross-sectional study did not permit assessment of changes in the incidence of thyroid disorders over time. Moreover, due to the minor population with thyroid diseases, the risk of bias should be considered to some extent. Additional cases should be enrolled for further discussion.